1. Field of the Invention
The present invention relates to optical tomographic imaging methods and apparatuses, and in particular to an optical tomographic imaging method and apparatus employing optical coherence tomography in the fields of ophthalmic diagnosis and the like.
2. Description of the Related Art
In recent years, imaging apparatuses utilizing interference of low-coherence light have been in practical use.
In particular, an optical tomographic imaging apparatus based on optical coherence tomography (hereinafter abbreviated as OCT) in which multiple-wavelength interference is utilized provides a high-resolution tomogram of a sample. OCT apparatuses have been becoming essential, particularly in the field of ophthalmic diagnosis, for acquisition of a tomogram of the eyeground or a region therearound.
Other than in ophthalmic diagnosis, OCT apparatuses are also used in, for example, tomographic examination of the skin and, by being incorporated into endoscopes and catheters, in tomography of the walls of digestive and circulatory organs.
Such OCT methods include a time-domain OCT (TD-OCT) method. Examples are disclosed in U.S. Pat. No. 5,321,501 and PCT Japanese Translation Patent Publication No. 2002-515593 (pages 9 to 19 and FIGS. 1 and 2 therein).
A TD-OCT method will now be described.
FIG. 2 is a diagram of a known TD-OCT apparatus.
Referring to FIG. 2, light emitted from a light source 2101 travels through an optical fiber 2102, is guided to a collimator 2103, and is output as a collimated beam 2104 from the collimator 2103.
The collimated beam 2104 is split into a reference beam 2106 and a signal beam 2107 by a beam splitter 2105.
The reference beam 2106 is reflected by a movable reference mirror 2111-2.
The movable reference mirror 2111-2 is moved by a reference mirror stage 2112 mechanically movable in a one-dimensional direction, thereby controlling the measurement point in an object 2117 to be examined. The measurement point is controlled in an optical-axis direction of the signal beam 2107 entering the object 2117.
The signal beam 2107 is directed by a scanning optical system 2108 in such a direction as to travel through eyepiece members 2109-1 to 2109-3, thereby entering the object 2117.
The beam that has entered the object 2117 is reflected by layers in the object 2117 and returns to the beam splitter 2105.
The scanning optical system 2108 performs scanning by moving the signal beam 2107 entering the object 2117 in specific directions.
The reflections from the movable reference mirror 2111-2 and the object 2117 enter the beam splitter 2105 and interfere with each other, producing an interference beam 2113. The interference beam 2113 is condensed by a collimator 2114 and is guided to a detector 2115.
The condensed beam is detected as a signal by the detector 2115 and is imaged by a control computer 2116.
The control computer 2116 controls the eyepiece member 2109-2 by moving a focusing stage 2110 so as to adjust the focus of the beam incident on the object 2117.
The control computer 2116 also controls the reference mirror stage 2112, thereby being capable of identifying, with reference to the detected interference signal and the position of the reference mirror stage 2112, the depth in the object 2117 indicated by the detected signal.
Thus, in the TD-OCT method, image data can be generated from pieces of data representing the intensity of the interference beam, the pieces of data being successively acquired by performing scanning with the scanning optical system 2108 while controlling the movable reference mirror 2111-2.
For example, in A-scan (scanning in the optical-axis direction of a beam entering an object, i.e., the depth direction of an object), the signal beam 2107 entering the object 2117 is moved by the scanning optical system 2108 in one direction (for example, the X-axis direction) within a plane in the object 2117.
Thus, pieces of one-dimensional A-scan data can be successively acquired.
From such successively acquired images, a B-scan image (a two-dimensional image of a longitudinal section) can be acquired.
If the signal beam 2107 is moved in two directions (for example, the X- and Y-axis directions) within the foregoing plane without moving the reference mirror stage 2112, a C-scan image (a two-dimensional image of a transverse section) can be acquired.
If the signal beam 2107 is moved in two directions (for example, the X- and Y-axis directions) within the foregoing plane while the reference mirror stage 2112 is controlled, a three-dimensional image can be acquired.
Here, A-scan, B-scan, and C-scan mentioned above will be described with reference to FIG. 3.
A signal beam 3207 is made to enter an object 3217 to be examined, as shown in FIG. 3.
The signal beam 3207 is made to enter the object 3217 in the Z-axis direction in FIG. 3. Therefore, in A-scan, information on a structure extending along an axis 3218 in the object 3217 is acquired.
If, in FIG. 3, the signal beam 3207 is moved in the X-axis direction while scanning in the Z-axis direction is performed, information on a plane 3219 is acquired.
The plane 3219 is a longitudinal sectional image. A scanning method providing such an image is referred to as B-scan.
If scanning is performed in the X- and Y-axis directions but not in the Z-axis direction, information on a plane 3220 is acquired.
The plane 3220 is a transverse image of a layer in the object 3217. A scanning method providing such an image is referred to as C-scan.
There is another OCT method, a spectral-domain OCT (SD-OCT) method. An example is disclosed in “Handbook of Optical Coherence Tomography” (2006; FIGS. 2 and 3 in pages 145 and 149, and FIG. 1 in page 338).
The SD-OCT method will now be described.
FIG. 4 is a diagram of a known SD-OCT apparatus.
The SD-OCT apparatus shown in FIG. 4 differs from the TD-OCT apparatus shown in FIG. 2 in the following aspects.
The SD-OCT apparatus includes a fixed reference mirror 4323 instead of the movable reference mirror 2111-2, spectroscope members 4321-1 and 4321-2 including a diffraction grating, and a spectroscopic detector 4322, such as a line sensor, instead of the detector 2115.
In the SD-OCT apparatus, a spectrum acquired by the spectroscope members 4321-1 and 4321 is detected by the spectroscopic detector 4322.
In the SD-OCT apparatus, the detected spectrum, which is composed of pieces of information on the intensity of an interference beam 4313 expressed with respect to an axis representing the wavelength, is subjected to a Fourier transform into information expressed with respect to an axis representing the position of the scanned plane, whereby image data is acquired collectively in terms of time.
In the SD-OCT apparatus, since image data in the depth direction of an object 4307 is collectively acquired, measurement speed can be increased compared with the case in the TD-OCT apparatus in which scanning in the depth direction is performed successively in terms of time.
It is known that SD-OCT apparatuses can acquire B-scan images and three-dimensional images at higher speeds than in TD-OCT apparatuses. Instead, it is pointed out that there is a difficulty in increasing the transverse resolution in SD-OCT apparatuses.
This is because, in SD-OCT apparatuses, the measurement range of an object in the depth direction thereof is inversely proportional to the transverse resolution. Therefore, to acquire information on a longitudinal section having a sufficient area, the transverse resolution needs to be reduced.
In recent years, there has been a demand for OCT apparatuses having increased image resolutions.
There has also been a strong demand for capability of concentrated examination of an arbitrary layer in an object, and accordingly a demand for apparatuses capable of taking C-scan images with high resolutions and at high speeds.
The known TD-OCT and SD-OCT apparatuses described above, however, have the following problems in satisfying such recent demands.
For example, the known TD-OCT apparatus described above can acquire a two-dimensional image of a transverse section by C-scan in which the signal beam 2107 is moved in two directions (for example, the X- and Y-axis directions) within a plane without moving the reference mirror stage 2112.
In this operation of the TD-OCT apparatus, only a signal from a specific depth defined by the movable reference mirror 2111-2 is acquired. Therefore, by focusing the signal beam 2107 on the specific depth, a transverse-sectional image with a very high resolution can be acquired.
In this case, however, since the signal beam 2107 is moved with the movable reference mirror 2111-2 fixed, only a two-dimensional image of a transverse-sectional image at a constant depth is acquired.
Therefore, in a case where a desired object to be examined is a curved layer, such as an internal structure of a retina, only fragmental transverse-sectional images of the desired layer are acquired.
Another method can be considered in which three-dimensional information is generated from successively acquired B-scan images, and a surface of an arbitrary desired layer is extracted from the three-dimensional information. Such a method, however, requires a long time for acquisition of three-dimensional information. Besides, if the object moves during acquisition of such information, the resulting image may blur and clear information may not be acquired.
In addition, to generate three-dimensional image information, high-load processing needs to be performed. This leads to a problem of a very long processing time.
On the other hand, in the SD-OCT apparatus, information in the depth direction over a wide range can be acquired collectively, whereas it is difficult to increase transverse resolution.
If the transverse resolution of the SD-OCT apparatus is increased, the longitudinal dimension of a range in which information can be collectively acquired is reduced, and the speed of information acquisition is also reduced to a level equal to or less than that in the TD-OCT apparatus.
Moreover, to acquire information on an arbitrary surface, three-dimensional information first needs to be generated from a plurality of B-scan images, and then an image of the surface needs to be extracted, as in the TD-OCT apparatus.
Such acquisition of high-resolution three-dimensional information also requires a very long time. Besides, if the object to be examined moves, the resulting image may blur.
In addition, it takes a very long processing time, as in the TD-OCT apparatus.